1. Field of the Invention
The present invention relates to a cathode ray tube used in television receivers, computer displays or the like that have a color selection mechanism such as a shadow mask.
2. Description of the Related Art
A conventional cathode ray tube will be described with reference to FIG. 4. A glass envelope includes a panel 4 for forming a screen, a funnel 6, and an electron gun 8 provided in a neck portion 6a of the funnel 6. A phosphor plane 1 formed of phosphors of three colors of red, green and blue and an aluminum layer 2 are formed on the inner surface of the panel 4. Further inside thereof, a shadow mask 3, which is a color selection mechanism, is provided at a predetermined distance from the inner surface of the panel 4. An internal conductive layer 5 is formed on the inner surface of the funnel 6, and a high voltage (about 30 kV) is applied to the conductive layer 5 from the outside through an anode button 9. This high voltage is applied to an electron gun 8 through the internal conductive layer 5, and also is applied to the shadow mask 3 and the phosphor plane 1. Thus, an equipotential space of the high voltage is formed from the electron gun 8 to the phosphor plane 1.
Next, the operation of the cathode ray tube will be described. Electron beams are released from the electron gun 8, and images are formed on the phosphor plane 1. A deflection yoke 12 provided in a portion between the funnel 6 and the neck portion 6a deflects the electron beams in horizontal and vertical directions. When the electromagnetically deflected electron beams leave the electromagnetic deflection region, the beams follow a straight trajectory and reach the shadow mask 3. Then, the electron beams pass through the apertures of the shadow mask 3, and impinge on the phosphors on the phosphor plane 1, so that the phosphors are excited for light emission. When scanning the periphery of the screen, the electron beams 15 are incident obliquely to the shadow mask 3 with an angle of xcex8m.
In order to reduce the total length of the cathode ray tube, it is necessary to enlarge the deflection angle xcex81, i.e., the incident angle xcex8m to the shadow mask 3, in the conventional system. For this reason, significantly high precision is required for positioning the deflection yoke 12 when it is mounted.
Furthermore, since about 80% of the electron beams impinge on the shadow mask 3, a so-called doming phenomenon, which is a thermal expansion phenomenon, is caused, so that the displacement in the position of the apertures causes mislanding of the electron beams. The larger the incident angle xcex8m is, i.e., the larger the deflection angle xcex81 is, the larger is the possibility of even a small displacement of the aperture position causing a large mislanding. Also with respect to the influence of geomagnetism, a large angle xcex8m causes more significant mislanding.
In order to solve these problems, the following approach has been proposed. A different desired electric potential is applied to at least a part of the electron gun and the funnel from that applied to the screen portion, so that an electrostatic lens is formed between the funnel and the screen portion, or a further electrostatic lens is formed between the electron gun and the funnel or in other portions. This configuration prevents the electron beams deflected by the deflection yoke from traveling along a straight trajectory when they leave the electromagnetic deflection region, and allows the electron beams be incident to the shadow mask at an angle xcex8m smaller than the angle xcex81. Such a configuration where an electrostatic lens is formed in the tube is disclosed in, for example, JP 55-1012 A. In this disclosure, instead of the conductive layer 5 being applied uniformly onto the inner wall of the funnel, a conductive layer made of graphite is applied independently to a plurality of regions, and different electric potentials (25 kV as a high electric potential and 12 kV or 17 kV as a low electric potential) are applied from region to region, so that electrostatic lenses are formed in the cathode ray tube.
However, in the above-described conventional configuration, the conductive layer is formed of porous graphite having a small specific resistance. Therefore, discharge tends to occur between the conductive layers having different electric potentials in a vacuum. In order to keep the electric potential of each of the conductive layers independent, it is necessary to enlarge the distance between the conductive layers. This leads to a large exposed area of the glass base material, so that problems such as the distortion of the electrostatic lens, discharge and charge drift are caused.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a highly reliable cathode ray tube where there is no interference of at least two different electric potentials due to discharge or the like, and the electric potentials are kept independent and stable. A cathode ray tube of the present invention includes a bulb including a front panel and a funnel-shaped member (funnel), and an electron gun. The front panel includes a color selection mechanism and phosphors on an inner surface thereof. The funnel includes at least two conductive layers on an inner wall thereof. The electron gun is accommodated in a neck portion of the funnel. The conductive layers have different electric potentials from each other. The conductive layers include a high electric potential layer having a high electric potential and a low electric potential layer having an electric potential lower than that of the high electric potential layer. An insulating region is formed between the high and low electric potential layers. A resistive layer is further formed between the low electric potential layer and the insulating region.
This embodiment prevents at least two different electric potentials from interfering with each other because of discharge or the like in the cathode ray tube and allows the electric potentials to be independent, and the conductive layers can supply stable electric potentials. Thus, an electrostatic lens can be formed.
Alternatively, the resistive layer can occupy a region between the low electric potential layer and the high electric potential layer, instead of the insulating region being formed.
This embodiment prevents at least two different electric potentials from interfering with each other because of discharge or the like in the cathode ray tube and allows the electric potentials to be independent, and the conductive layers can supply stable electric potentials. Thus, an electrostatic lens can be formed. Furthermore, since the glass surface of the funnel is not exposed, the problem of charge drift can be solved.
Furthermore, it is preferable that the resistive layer and electron beams are insulated by an internal magnetic shield, and the internal magnetic shield has the same electric potential as that of either one of the panel or the funnel.
This embodiment prevents the glass surface of the funnel or the resistive layer in the cathode ray tube from being exposed to electron beams. Therefore, problems such as distortion of raster shape due to a coating shape of the region and charge drift can be solved.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.